BackgroundCancer incidence increases with age, so some clinical guidelines include patient age as one of the criteria used to decide whether a patient should be referred through the urgent suspected cancer (USC) pathway. Little is known about how strictly GPs adhere to these age criteria and what factors might influence their referral decisions for younger patients.AimTo understand GPs’ clinical decision making for younger patients with concerning symptoms who do not meet the age criteria for USC referral.Design and settingQualitative study using in-depth, semi-structured interviews with GPs working in surgeries across England.MethodParticipants (n= 23) were asked to recall consultations with younger patients with cancer symptoms, describe factors influencing their clinical decisions, and discuss their overall attitude to age thresholds in cancer referral guidelines. A thematic analysis guided by the Framework approach was used to identify recurring themes.ResultsGPs’ decision making regarding younger patients was influenced by several factors, including personal experiences, patients’ views and behaviour, level of clinical concern, and ability to bypass system constraints. GPs weighted potential benefits and harms of a referral outside guidelines both on the patient and the health system. If clinical concern was high, GPs used their knowledge of local systems to ensure patients were investigated promptly even when not meeting the age criteria.ConclusionWhile most GPs interpret age criteria flexibly and follow their own judgement and experience when making clinical decisions regarding younger patients, system constraints may be a barrier to timely investigation.